Lasers and other coherent light sources are attractive illumination sources for optical microscopes and other image-forming systems due to their high brightness and monochromaticity. However, there are two primary limitations to the use of such sources which must be overcome if high-quality images are to be obtained. First, the images derived from systems illuminated with spatially and temporally coherent radiation often exhibit a psuedo-random interference pattern of light and dark regions across the entire field of view. This interference pattern is known as speckle and is due to the interference of the coherent light scattered from different points on the object. The speckle pattern is highly undesirable if quantitative information is required from the images, e.g., transmission measurements of a partially opaque object.
Further, the beams derived directly from laser sources often exhibit a spatial intensity distribution which is substantially non-uniform. For example, many laser systems are characterized by a Gaussian-shaped intensity distribution. High-quality images generally require a homogenized, uniformized illumination source, i.e., a circular, “flat-top” intensity distribution. Often, these beams have additional small-scale intensity structure due to dust spots on mirrors, &c., which would mar the quality of the final image if not removed or compensated.
Many means have been proposed to homogenize the spatial intensity distribution and/or reduce the speckle contrast. Speckle reduction techniques generally fall into two categories: reduction of illumination source spatio-temporal coherence, or time-averaging of dynamic speckle patterns. Occasionally, the methods used to reduce the speckle also act to homogenize the spatial intensity distribution. The first approach to speckle reduction reduces the optical coherence of the illumination source to the point that there is insufficient interference to produce visible speckle patterns. The second approach actually takes advantage of the coherence properties to produce speckle patterns which vary in time and which may be combined via temporal integration and/or averaging until the speckle contrast is reduced to an acceptable level. To a large extent, the choice of homogenization and speckle reduction technique depend on the exact parameters of the illumination source itself, including power levels, wavelength, and repetition rate.
A setup popular in semiconductor optical lithography to improve beam homogeneity with low-coherence sources has been a fixed length of transparent solid rod [see K. Jain, Excimer Laser Lithography, SPIE Press, Bellingham, Wash., 1990: pg. 114]. Light enters one side of the rod at a variety of angles, and then propagates through to the other side via a number of total-internal-reflection (TIR) bounces. This setup effectively removes any intensity variations from the incident beam. Additionally, the path length of each ray depends upon the incidence angle and exact shape of the rod, so that the phase profile is also scrambled. A natural extension of this approach has been the use of a single, fixed, multi-mode optical fiber, through which the illumination light propagates. Both apparatus are suitable only for already low-coherence sources.
Elimination of the optical coherence is an attractive choice because no temporal averaging is required, hence throughput is increased, but is often prohibitively difficult. One manner in which the spatial coherence may be reduced is to divide the output of the illumination source into several portions, each of which is made to travel a slightly different path length to the imaging plane of the microscope. If the difference in path lengths between these separated portions is on the order of, or greater than, the coherence length of the source, then upon recombination of the separated portions, the light will seem to originate from a number of separate sources with no fixed phase relationship. The division into multiple ‘independent’ sources also can provide homogenization of the spatial mode of the illumination radiation if each of the beams passes through different aberrating media. The degree of improvement in the final images depends upon the number of ‘independent’ sources that are created. The optical arrangement to perform this operation using discrete optical components is quite bulky and suffers from low overall throughput, and is not suitable for long-coherence length sources, e.g., single-frequency lasers.
Consider now the second approach to speckle reduction, that of averaging dynamic speckle patterns. This approach is suitable for high-coherence sources such as single-frequency lasers. If the phase profile of the illumination beam can4 be altered dynamically without destroying the coherence, then although individual snapshots taken by the imaging system may exhibit high-contrast speckle patterns, an integrated image of several randomly-oriented speckle patterns will appear smooth. Perhaps the simplest setup historically has been a single or double set of ground-glass diffusers inserted into the illumination beam path. The surface of these diffusers, on the scale of an optical wavelength, is a random hilly pattern due to the random process of grinding. Thus, different points on the diffuser are of slightly different thicknesses, and different rays travelling through different regions of the diffuser will pick up psuedo-random phase shifts relative to one another. (However, since the size of these small hills is much less than the coherence length of the source, the phase profile is still coherent.) The randomized phase profile subsequently creates a dynamic, high-contrast speckle pattern in the image plane of the optical system which is recorded along with the desired test sample image. Additionally, the rough surface efficiently scatters the incident illumination light into a broad range of angles, so that each illumination point consists of contributions from many points on the incident illumination beam. Thus both requirements, i.e. spatial beam homogenization and the creation of a dynamic speckle pattern, are met. The disadvantage of this method is that the diffusers (since they scatter light so effectively) are extremely lossy; often less than {fraction (1/1000)}th of the available illumination light is available for actual imaging. This problem precludes use of the diffuser technique at difficult-to-produce illumination wavelengths, especially in the vacuum ultraviolet (VUV).